I have a financial situation where I am receiving a cash inflow first, then
in the next 4 years will only be outflows. How do I properly do an IRR
calculation with this? Or can I even do it?

I can tell the value that comes out of the IRR function is not correct. Is
it as simple as taking the opposite of the IRR value excel gives me?
Something tells me no, but I'm not sure.

Simple example would be

Year 1 - inflow 1500
Year 2 - outflow 250
Year 3 - outflow 250
Year 4 - outflow 250
Year 5 - outflow 250

This tells me IRR is -14.3% which is not correct.

Thanks in advance.